Aircraft power systems



y 7, 1959 c. J. MODOWALL ETAL 2,893,525

AIRCRAFT POWER SYSTEMS Original Filed Dec. 12. 1952 3 Sheets-Sheet 1 Inventors Attorney July 7, 1959 c. 1. MODOWALL ET AL ,5

AIRCRAFT POWER SYSTEMS Original Filed Dec. 12, 1952 3 Sheets-Sheet 2 Attorney Inventor:

July 7, 1959 c. J. MCDOWALL ETAL 2,893,525

- AIRCRAFT POWER SYSTEMS Original Filed Dec. 12,1952 3 Sheets-Sheet 3 Attorney 2,893,525 lc Patented July 7, 1959 2,893,525 I AIRCRAFT POWER SYSTEMS Original application December 12, 1952, Serial No. 325,562. Divided and this application April 14, 1954,

Serial No. 423,024 I 8 Claims. (Cl. 192-48) This invention relates to power plants and more particularly to the transmission of power between an aircraft engine or engines and a propeller.

The invention is a division of Serial Number 325,562, filed December 12, 1952 and is particularly applicable to aircraft power plant arrangements wherein a plurality of gas turbine engines of comparable power are coupled by clutches to a common propeller or propellers, although the principles of the invention may also be applied to single engine arrangements and to the transmission of power in many fields other than the field of aircraft propulsion. Multiple engine power plant arrangements are especially useful in aeronautics as best efiiciencies during some flight conditions are obtained with one or more of the engines uncoupled and inoperative. Multiple engine arrangements are inherently safer than single engine arrangements and are therefore preferred for some types of aircraft operation. 7

When two gas turbine engines of comparable power are each clutched to a common propeller or propellers and there is a failure in one of the engines, it is essential that the failed engine be rapidly declutched to free the other engine. If the failed engine is allowed to remain coupled to the propeller, the failed engine will ordinarily absorb power from the propeller in addition to absorbing the entire output of the good engine. Under these conditions, the propeller moves into low pitch in an effort to maintain speed-and it absorbs power from the forward motion of the aircraft to help drive the failed engine. The resulting drag on the aircraft is likely to cause the pilot to lose control before he can declutch the failed engine. The danger of control loss is even more pronounced in a single engine arrangement. The drag resulting from a failed gas turbine engine is tenfold that of a comparable piston engine and the failure of a gas turbine, unlike the failure of a piston engine, is difficult to detect on the normal engine supervisory instruments provided for the pilot.

The invention provides for individually clutching each engine to the propeller by two clutches in series, one clutch being a hydraulically operated friction type clutch that is normally under manual control and the other being a normally engaged positive jaw type safety clutch that is automatically disengaged by substantial reverse torque, which disengagement automatically disengages the friction clutch. Either engine may be started with the friction clutch disengaged, and may then be clutched by it to the propeller. With one engine operating, the other engine may be clutched by its friction clutch to the operating engine and started thereby, either on the ground or in flight. Also, the power sections maybe started in flight by clutching them to the propeller to derive power from the windmilling of the propeller.

The principal objects of the invention are to provide an improved power plant of the turboprop type, particularly one with multiple power units; to provide improved re duction gearing, clutching, and starting arrangements for such power plants; to improve the control systems for starting such power plants and clutching them to the load; and to provide an improved clutching arrangement wherein the high drag of a failed power unit will effect a rapid uncoupling of the same from the propeller to prevent a loss of control over the aircraft.

The preferred manner in which the stated objects are achieved, the advantages of the invention, and many further objects of the invention will be apparent to those skilled in the art from the appended description of the preferred embodiment of the invention.

Referring to the drawings:

Fig. 1 is a somewhat schematic representation of the general arrangement of a power plant incorporating the invention;

Fig. 2 is a partial sectional view of the power transmission structure, the section being taken along the axis of one of the engine shafts and particularly illustrating the clutching mechanisms;

Fig. 3 diagrammatically illustrates the operation of the safety clutch, and

Fig. 4 is a schematic diagram of the invention.

Referring to Fig. 1, this figure illustrates a power plant comprising two gas turbine engines or power units A and B arranged side by side and coupled through a reduction gear transmission C to the variable pitch propeller D. The details of the internal structure of the engines and propeller are immaterial to the invention. Each of the engines is of a known type comprising a turbine to power the propeller, a combustor to deliver hot gas to the turbine to drive the same, and a compressor driven by the turbine to deliver air to the combustor for fuel admixture and combustion. The propeller is of a known type having a variable pitch control including feathering control.

The engines A and B are of the same power rating and deliver power to the propeller D through identical clutching arrangements. Each engines power output shaft 10 connects to the common propeller bull gear 12 through a safety clutch 14, an intermediate shaft 16, a friction clutch 18, a pinion shaft 19, and a pinion 20. A starter motor 22, including an overrunning clutch or the equivalent, drives a shaft 24, on which is splined a sliding toothed clutch element 26, which may be engaged with mating clutch elements of either of two gears 28 and 30, rotatable with respect to the shaft 24. The gears 28 and 30 are coupled to gears 32 on the intermediate shafts 16 through gears 33, 34 and 35.

Referring additionally to Fig. 4, the starting clutch 26 may be shifted to couple the starter with either gear 28 or gear 30 by appropriate mechanism such as a sliding fork 36 actuated into either extreme position by solenoids 38 and 40, respectively, which may be energized by a suitable circuit (not shown). The armature of solenoid 38 may be coupled directly to the shifting fork by rod 42 and the armature of solenoid 40 may be coupled to the rod 42 by a lever 44. Au overcenter detent mechanism (not illustrated) may be provided to maintain the clutch,

26 in either position of engagement.

The friction clutches 18 are engaged and disengaged by a hydraulic system and are supplied with oil for cooling. Oil for both these purposes is supplied by pump 46 supplied from the sump of the transmission housing 48 through a conduit 50. The pump 46 may also supply oil to the reduction gear and to the safety clutches 14. The pump 46 is driven by the starter shaft24 or the propeller drive shaft 52, being coupled to both these through overrunning clutches and suitable gearing. The illustrative gearing shown comprises bevel gears 54 and 56 coupled to the pump shaft 58 by overrunning clutches 60 and 62 respectively, gear 54 being driven by a bevel gear 64 on the starter shaft 24 and gear 56 through a gear train comprising gear 66 on shaft 52 and intermediate gears 68 and 70 on an idler shaft. The ratios of the gears aire"preferably "s selected that when the pr'biseuer" is V clutched to the power unit the pump is driven by the propeller and clutch 60 overruns slightly. The pump 46 supplies oil to a friction clutch actuating valve 270 and a friction clutch 'cooling'valve 272 for each engine through conduit 274'andsuitable valves and conduits'to be de scribed.

Before proceeding to the description of 'these valves and the control systems therefonthe structure of the c'lutches 18 and 14 will'be'described with referenceparti'cularly'to Fig. 2. Fig. 2 illustrates the clutching arrang'ement of either of the engines'and includes the clutch 14 which couples theengine output shaft to the intermediate shaft 16 which, in turn, is coupled by the clutch 18 to the pinion shaft 19 to'drive the propeller. The starter gear 32 which is splined to shaft 16 and a portion of the'idlergear 35 are shown.

The clutching arrangements are supported bythe housing"48,shown' more fully in Fig. l, which includes the clutch housing 72, the 'reductionlgear housing 74'and a transverse plate'7 6 bolted between the housings 72 and 74. The pinion 20 is integral with the hollow pinion shaft 19 which i's'mounted in a bearing78 in theplate 76 and a suitable bearing in the housing 74. The driven member 80 of the clutch 18 is in the form of a drum and is'provided with internal splines 82. The member 80 isformed with a flange that is' splined to'the shaft 19 at 84 and retained thereon by the threaded ring 86. The shaft 19 is formed with a reduced end portion on which is mounted a bearing 88 that supports the driving clutch element 90 rotatably on the shaft 19. The driving clutch element 90 is of'cylindrical form and is provided with splines'92 on its outer surface which constitutes an enlarged end portion of'the intermediate shaft'16. Shaft 16 is further mounted in a bearing 94 at'the end wall of the housing 72.

The annular piston 96 of the clutch actuating cylinder has a sleeve portion 98 piloted on the shaft 16 which abuts the end'of the driving clutch element 90 and sleeve 99 splined on the shaft 16'at'100 and 102. The sleeve 99 is in'turn abutted'by the inner race of the bearing 94, and thewhole assembly is retained on shaft 16 by a spanner nut 104. The starter gear 32 is integrally formed on the sleeve '99 to rotate with the intermediate shaft 16. The intermediate shaft 16 projects through the rear wall of the housing 72 and supports the safety clutch 14 to be described.

The clutch 18 is of the multiple-disk type and is provided with two sets of disks disposed alternately, the disks 106of the driving set being formed with internal splines that are engaged with the splines 92 of the driving clutch element'90, and the driven clutch disks 108 being formed with extefnal splines that are engaged with the splines 82 of the driven clutch element 80. .The innermost of the driving clutchdisks is backed up by a ring 110 which bears against an annular flange 112'threaded on the forwardend of the driving clutch element 90. The clutch disks arepressed into engagement by an annular cylinder 114 which reciprocates on the outer surface of the sleeve 98. 'Cylinder 114 is fitted with a head 116 and the ends of'the cylinder are provided with oil seals 118 engaging the sleeve 98. The cylinder 114 is divided into two chambers 120 and 122 by the fixed piston 96 which is provided with oil seals 124 around its circumference. As will be apparent, when oil'under pressure is admitted to the front chamber 120 the clutch disks are engaged; and when oil admitted to the rear chamber 122 the clutch is released.

With the safety clutch 14 in positive engagement and the hydraulic clutch disks in frictional engagement, power is transmitted from the engine output shaft to the intermediate shaft 16 and the driving clutch member 90 thereon'and through'the frictional contact of the disks to the driven clutch member 80, pinion shaft 19 on which it mounted, andthe' pinion 20 to drive'the bull gear'12 which powersthe propeller D. When in flight, the rotation of the propeller may drive back at low reverse I torq uethroughthe clutches 18 and 14 to bring the engine up to starting-speed, but may not drive back through the clutch 14 at high reverse torque, as will be seen.

Hydraulic operating fluid for the friction clutch is supplied through a plug fitting 126 pressed into the end of the shaft 16 and concentric slip tubes 128 and 130 extending from the fitting 126 to a non-rotating bushing (not shown) in the forward wall of the gear'hou'sing 74. The plug 126 rotates with the shaft 16 and the tubes float in the plug and gear housing 'busliing' mp'ermit free rotation of 'thesliaft. Oil to release the clutch 1s is'e snauted through the tube 128a'nd radialp'as'sag es 132m the plug 126, 134 in the shaft 16 and 136in the piston sleeve-98. Circumferential groovesare provided-in the outer surface of the plug 126 and the inner surface'of the sleeve98. Oil to engage the clutch 1 8istransrnittedthrough the tube 130 and radial passages 138, 140 and 142 in the plug 126, shaft 16 and piston sleeve 98, respectively, in a similar manner.

r An annular fitting 144 is secured on the rear stub'end of the plug 126 by a C-ning 146. The fitting I44 and the plug form annular chambers 148 and 150 which connect by longitudinal passages 152 in'the fitting. A ring valve 154 is reciprocably received in the chamber 148 and is ing the'oil supply to' the safety clutch bearing.

Cooling oilfor the clutch 18 is supplied through'the slip tube 164, one end of which floats in a recess in theplug 126 and the other" end of which floats in a non-rotating bushing (not shown) in the forward wall of the gear housing 74. Passages 166 in the tube 164 discharge the cooling fluid into an annular chamber 168 defined by the driving clutch member 90, the plug 126, and a disk 170 mounted in the driving clutch member. When oil issupplied to the chamber 168, centrifugalforce drives the oil through radial passages 172 in the driving clutch-member which discharge the oil between the clutch disks-106and 108 which maybe surface grooved in a'known manner. The driven clutch member 80 is formed with outlets 174 through which the oil' is vented into the oil pan 176 for chamber only radially outward from the outlet.

transfer to the oil sump.

The disengagement chamber 122 is formed with arestricted vent outlet 178 spaced from the sleeve 98. After the disengagement oil is shut off, the chamber 122 is partially drained through the orifice 178, leaving fluid in this This diminishes the centrifugal effect against the cylinder head 116 thus increasing the clutch-engaging force.

The cylinder head 116 is also provided with a'springloaded relief valve 180 which further vents the oil from the disengaging chamber 122 when the assembly approaches full speed as, for example, at'14, )()0 rpm. The relief valve may be of conventional construction and may be mounted in a boss on the cylinder head. In its preferred form, the valve' comprises aball '182seating against a radial opening 184 in the cylinder head and held seated by a spring 186 abutting a retainer 188. When the centrifugal force reaches'the desired level, the combination of centrifugal force on the valve parts and' on the oil lifts the ball 182 from theseat and vents the oil through the passage 184 and an outlet 190 in the valve body. The entire chamber 120 is filled with oil but'only the outer portion of the chamber 122 is filled when the friction clutch is engaged so unbalanced hydrostatic force due to centrifugal action helps to hold the 'clutch'engaged. The parts are so proportioned that this effect is sufficient to retain the clutchIS engaged during normal assent operation even though the pressure in the hydraulic fluid supply system be lost.

The safety clutch 14 provides an immediate separation between a failed power section and the propeller bull gear whenever a reverse torque above a preset value is applied to the failed power section. The separation or disengagement of the safety clutch actuates a switch 240 which effects disengagement of the failed engines friction clutch and shuts off the failed engines fuel supply. The .engine output shaft is suitably bolted to the safety clutch driving coupling member 200 at 201. -The driving coupling member includes an outer annular member 202 and a head member 204 secured together by the pins 206 and C-ring 208. A driven coupling sleeve 210 is slidably received in the interior of the driving coupling member 200 and is splined to the intermediate shaft 16 at 212 for axial movement thereon. The driving member 200 and the driven sleeve 210 are normally clutched together by helical splines 214 which comprise internal jaw teeth 216 on the head member 204 and external jaw teeth 218 on the sleeve 210,'as may be seen diagrammatically in Fig. 3. Three conical spring washers or Belleville springs 220 press against an annular cam plate 222 mounted on the driven sleeve 210 to keep the jaw teeth positively engaged except in the event of an engine failure at operational speeds. 1

The particular safety clutch illustrated is adapted for engines having counterclockwise rotation (looking at the engine from its rear end) so left-hand helical splines are provided. When power is transmitted through the safety clutch from the engine, the cylinder 200 and sleeve 210 rotate as a unit. When power is transmitted through the'safety clutch to the engine, the helical splines 214 generate a separating force proportionate to reverse torque which force tends to move the sleeve 210 forwardly against the resistance of the Belleville springs 220. If the reverse torque is of sufiicient degree, the jaw teeth 218 move forwardly and out of positive clutching engagement with the jaw teeth 216. Fig. 3 illustrates anintermediate point in such a disengaging movement. Ratcheting occurs between the overriding ends 224 and 226 of the jaw teeth 216 and 218 when they move out of positive clutching engagement with each other. The overriding ends 224 and 226 are relieved with a suitable ramp angle to facilitate ratcheting. In the particular helical spline arrangement illustrated, the teeth faces are formed with a 28-degree helix angle and the overriding ends of the teeth are formed with an S-degree ramp a f The amount of reverse torque that maybe transmitted through the safety clutch without decoupling depends primarily upon the helix angle of the jaw teeth and the: resistance offered by the Belleville springs. The helix angle and spring force are so related that a substantial amount of reverse torque may be transmitted through the clutch Without eifecting its decoupling so that reverse starting torques may be transmittedto the engine. Certain flight situations also may generate reverse torques through the clutch which should not eifect its decoupling; for example, high speed landings at relatively low engine power settings, because of propeller windmilling. These;

reverse torques wherein decoupling is not desired are. substantially less than the reverse torques that occur: engine failures at operational speeds and may therefore be accommodated without compromising the safety function of the clutch. I A

The particular arrangement illustratedis designed to decouple a failed engine from the power-plant whenthe failed' engine absorbs approximately of its. rated horsepower at operational speed. The decouplingsetting may,rof course, be varied in accordance with the requirements of any particular power plant, but it-should be at least high enough that -starting torques may be, transmitted. An axially adjustable abutment for thBelleville springs is Provided to compensate for dimensional 'variations of the parts as manufactured, and it should be obvious that the decoupling setting may be varied by changing the number of springs and/or helix angle.

The inner rims of the Belleville springs 220 abut against an inner sleeve 228 that is externally threaded to an internally threaded outer sleeve 230. The outer. sleeve 230 abuts against the axially projecting teeth 232 of the spanner nut 104 and the sleeves are provided with locking tangs 234 and 236 that engage between the nut teeth 232. The sleeves 228 and 230 comprise an axially adjustable abutment for minor adjustments of the preloading setting of the Belleville springs. The helical splines 214 are preferably loaded by Belleville springs because their spring force diminishes as they flatten out and overcenter under the separating force generated by the splines. This force-diminishing characteristic of the Belleville springs insures a rapid decoupling of the safety clutch whenever design decoupling reverse torque is attained and considerably reduces spring loading during that ratcheting will occur at the minimum value.

In the event of an engine failure, the high drag of the failed engine immediately effects automatic decoupling of the safety clutch 14 which ratchets and allows the engine to coast to rest. The sleeve 210 is shifted sufficiently during ratcheting to cause the outer surface of the cam plate 222 to engage the reciprocable lever 238 of the microswitoh 240 to actuate the same. The microswitch 240 transmits an electrical signal to shut off the fuel to the failed engine through a suitable valving arrangement (not shown) and to disengage the failed engines friction clutch 18 through means to be described. The cam plate 222 may trigger the switch 240 slightly ahead of actual ratcheting, if desired, but ratcheting will still occur because of the time lag in declutching the friction clutch 18.

The safety clutch insures decoupling of the failed power section without reliance upon the electrical and hydraulic systems of the aircraft and provides for automatic declutching of the manually controllable clutch 18.

The friction clutch 18 is preferably disengaged by the that of the failed engine (which may be coasting to rest at low r.p.m. or be at rest) and the spring loading of the clutch 14 effects positive re-engagement of the jaw teeth. a

In some designs, the drag of the friction clutch 18 while running disengaged may exceed a desired low ratcheting drag of the safety clutch 14 and in such cases the remainder of the power plant may be momentarily shut down at an opportune time to effect re-engagement if a lengthy operation of the remainder of the power plant after a decoupling is desired. The safety clutch cam plate 222 may be adapted to engage a brake upon decoupling movement to augment the low ratcheting drag and eliminate the necessity for a shutdown in these designs, if desired.

The engine output shaft 10 is supported at its forward end by the safety clutch 14, the safety clutch is supported by the intermediate shaft 16, and the bearing 94 provides support on the rear wall of the clutch housing 72 for the intermediate shaft. The journal bearing 162 and the spline connection 212 support the safety clutch onthe intermediate shaft 16. The journal bearing 162 is secured.

in the shaft 16 by a C-ring, 242. .A small shaft 244 is secured to the cylinder head 204 by a spanner nut 2246 and is provided with a sleeve 248 and a flanged end 250 that 'eiiibrace'the"ends'; of the ournal bearing 16256 that the bearing may take both the radial and the thrust loads that result from' ratcheting of the safety clutch. The journal bearing 162 is lubricated whenever the power plant is operative by either engaging or disengaging oil from the friction clutch 18 through the radial passages 252 in the slip tube '163 and the radial passages 254 in the small shaft 244. The journal bearing 162 is so designed as to have sufficient oil leakage to fill the interior of the cylinder 200 during power plant operation, and the safety clutch is provided with sealing rings 256 and 258 so that the leakage oil may escape only into the'interior of the intermediate shaft 16 through an intermediate shaft vent'hole 260. The leakage oil is exhausted from the interior of theshaft 16 through a radial passage 262 in the shaft to lubricate the bearing 94 and an oil seal 264' of conventional design prevents the leakage oil from escaping out of the clutch housing 72. The leakage oil'in the cylinder 200 is under centrifugal pressure as a result of its rotation, and this pressure tends to move coupling 210 in opposition to the Belleville springs 220 as a resultof the unbalan'cedpressure on the coupling (the unbalanced pressure being determined by the thickness of the sleeve portion of the coupling). The coupling 210 is provided with radial passages 266 and 268 to circulate the leakage oil between the sides of the flange portion of the coupling.

The centrifugal hydraulic force is negligible with respect to the Belleville spring force (approximately one tenth thereof) when the safety clutch is'coupled. The centrifugal hydraulic force diminishes appreciably when the safety clutch ratohets for the cylinder 200 comes to rest with the engine shaft 10, but the coupling teeth 218 function as pump blading during ratcheting for the coupling 210 continues to rotate with the intermediate shaft 16 so that the net hydraulic force on the coupling remains substantially constant. Since the Belleville ring force during ratcheting is diminished to approximately one-fifth of its force while coupled, the ratio between the hydraulic force on'the coupling or piston210 and the spring force will be changed during ratcheting so that the hydraulic force is appreciable (approximately onehalf) with respect to the Belleville spring force thereby further reducing ratchetingdrag to prolong the life of the clutch teeth. I

Proceeding to a description of'the control system for the clutches with reference'to Fig. 4, it should be noted that the clutches 18 and 14 are shown in a schematic fashion in this figure. However, the relation of the schematic to the physical structure shown in Fig. 2 will be apparent to those skilled in the art, particularly in view of the identification of the principal elements of the clutches by reference numerals in Fig. 4.

As previously stated, the pump 46, which is'operated whenever either the power unit to which the starting shaft 24 is clutched or the propeller drive shaft 52 is rotating, supplies fluid to an actuating valve 270 and a cooling control valve 272 for each' of the clutching arrangements.

The controls for the two clutching arrangements are identical. It will be understood that the fluid supply system is provided with relief or unloading valves (not shown), which may be of conventional type, and that the pump 46 may supply lubricating oil under pressure to the-gears and hearings in any suitable manner in addition to supplying oil to the clutches. The valves 270 and 272 may be of the well-known spool type, as illustrated, or any other suitable type.

The line 274 from the pump branches and communicates directly with the coolant control valves 272. The engagement control valves 270 are supplied from line274 through a pressure-regulating valve 276 of any suitable type and branch conduits 278. The purpose of the regulatin g valve is to ensure substantially constant low oil pressure for soft engagement and disengagement By way of example, in the preferred embodiment of the inventionf'the' normal output of pump'46 is about 180 pounds per' square inch. 1' When cooling oil is supplied to the clutchthe' large flow of coolant reducesthe pressure'to about 40 or 50 pounds per square inch. The output of valve'276 is about pounds per square inch and is thus substantially independent of fluctuations in pump output pressure.

The system is illustrated in Fig. 4 in condition for starting oneunit with the clutches 18 disengaged, the clutches 14 engaged, and the starter 22 clutched to the intermediate shaft 16 of the power unit A. Under these conditions, flow of fluid from the pump is cutoff at the valves272 and is directed to the disengagement chambers anism. This mechanism includes a rod 282, guided for reciprocating movement, on which is mounted a plate 284. The plate 284 is formed with a fork engaging with lost motion between enlargements 286 and 288 of the plunger shaft 290 of valve 270. The plate 284 also engages the end of the plunger292 of the valve 272. Rod 282 is reciprocated through a pin and slot connection by a pivoted; lever 294 which may be actuated by the pilot in any suitable manner, as by a handle 296. I

When the lever 294 is'in the solid line position indicated by a, valve 270 is held in the clutch-releasing position in which fluid from the supply line 278 is supplied through the conduit 128 to the releasing chamber 122 of the clutch 18 and the clutch-engaging chamber 120 is vented through conduit 130 to the return line 280. In this position the plate 284 is clear of the stem 292 of the coolant supply valve 272 and this valve is held closed by a spring 298. The safety clutch oil supply valve 154 is held in the indicated position by the pressure in the conduit 128 and fluid is supplied through the conduits 128 and 163 to the journal bearing 162 and the cylinder 200 of the safety clutch 14.

To engage the clutch 18 of the power unit A, for example, the lever 294 is rotated to the position indicated as b, shifting the plate 284 to the b position. The connections to the chambers 120 and 122 are reversed, chamber 120 being filled through conduit 130 to engage the clutch 18 and chamber 122 being vented through conduit 128 to the open end of the valve 270 which may be connected to an oil sump return line (not shown).

The valve 154 is shifted to the right by the pressure in the conduit 130 and fluid is supplied through the conduits 130 and 163 to the journal bearing 162 and the cylinder 20-0 of the safety clutch 14. In the 1) position, the coolant valve 272 is opened against the force of the spring298 tosupply cooling fluid to the clutch 18.

When the propeller has been brought up to speed, the actuating lever 294 and thereby the plate 284 are shifted to the 0 position in which the spring 298 closes the valve 272 to shut off the cooling fluid. Valve plunger 290 is retained in the clutch-engaging position because of the lost motion between the plate 284 and the enlargements 286 and 288 on the plunger and the action of a detent mechanism 300 including a spring-urged detent 302 cooperating with the tapered surface 304 of the valve plunger 290. The safety clutch 14 continues to receive venting the clutch-engaging chamber through the conduit and supplying fluid under pressure to the releasing chamber'122 and the safety clutch 14 through the conduit 128.

As is apparent it is possible for the pilot or engineer assess? of the aircraftto' control" the clutch 18 directly by manual actuation of the levers294', and it will also be apparent that some form of remcte'control operating mechanism for the levers 294'could' be installed if desired. Since the details of such remote actuating means are not material to the present invention, it is' deemed preferable in the. interestof conciseness to omit a description" of such remote controls.

In practice, however, whether the operation of the levers 294 be byjdirect or by remote control, it is highly desirable to include automatic controls to actuate the friction clutchcontrollingvalves to disengage the friction clutches in response to the high drag or reversetorque thataccjompaniesan engine failure. i

The exemplary control system for each friction clutch (illustrated in Fig.4) comprises, the safety clutch actu-f ated microswitch Miland asolenoid 308. The micro- I switch 240" is closedby a forward" dec'ouplingmovement of the cam plate 222 to energize the solenoid 308 from a battery 310. The clutch control lever 294comi'ects to the solenoid armature 3121 by a pin and slotconnection and solenoid energi'zation actuates the controllever 294 to the clutch disengaging p'osition a. 'The solenoids 308 may be utilized to cut on each engines fuel supply through suitable valving (not shown) when energized, or separate solenoids may be provided if desired.

Although it is believed that'the manner of operation:

and characteristics fof the system disclosed herein will be apparent to those skilled in the art from the foregoing, the "operation may be reviewed briefly.

Assuming that both engines are standing 'idle, the

clutch 26 is shifted to couple the starter to either power unit. As the engine is accelerated by the starter, its operation becomes self-sustaining and it develops 'sufii cient power to assume the propeller load. The safety clutch remains engaged and the solenoid 308 de-energized while starting so the lever 294 may be operated to the b position to shift the valves 270 and 2.72 to engage the friction clutch and supply cooling oil thereto. When the propeller has beenacc'elerated to idle or intermediate speed, the lever is released and spring 298 shifts it to the c position to shut as the coolant, the valve remaining in the clutch-engaging position.

With one power unit in idle or intermediate speed operation, the friction clutch of the inoperative power unit is engaged by its lever 29 4 and the inoperative unit is brought up to speed by the operative unit 'through'the previously engaged friction clutch and, the intermediate. gearing. The safety cluch of 'theunit 'be'ingstarted re m "coupled as it is disengag n ybv amount of reverse torque greater than starting torque as previously noted. Theinoperative power unit may also ,be started by the starter-instead of the operativepower unit if desired. Disengaging reverse torque (might be generated if the friction clutch of the inoperative power unit were engaged with theoperative power unit at full speed but the engaging slip of prevent disenga i g mediate speeds. Starts with'tlie 'starter motor 22 are accomplishedwithoutsafety clutch disengagement as the power capacity of the starter motor is not large enough togenerate disengaging reverse torque. It might be noted againthat the amount of reverse torque required to disengage the safety clutch according to the invention is substantial as it is predicated upon an engine failure, for example, jamming of the compressor or turbine blading or a failure in the combustor fuel supply.

In flight, when power demand is reduced, either power unit may be declutched by its friction clutch and stopped, the propeller being driven by the operating unit. Both the power units may be shut down during flight (as might be desirable on a multi-powerplant aircraft) and restarted by unfeathering the propeller and allowing it to windmill freely to an intermediate speed whereupon the friction clutches may be engaged and the power units the friction clutch is suff cient to reverse torque generation at interbrought up to'firing-speed by the action of the apparent wind on the propeller and the blading of the power units.

In the event of a power unit failure, the failed unit absorbs sufiicient power from the other unit and/ or propeller to disengage its safety clutch and cut off its fuel supply. Disengagement of the failed units safety clutch effects automatic disengagement of its friction clutch and the" safety clutch then re-engages as the failed unit slows to rest, the compressor inlet of the failed unit being closed to prevent any windmilling of its blading.

The safety clutch prevents a loss of pilot control over the aircraft, especially during landings and take-offs, as

may result from the enormous drag of a failed engine 1 by automatically disengaging the same.

While the preferred embodiment of the invention has been described fully in order to explain the principles of the invention, it is to be understood that modifications in structure may be made by the exercise of skill in the art-within the scope of the invention, which is not to be regarded as limited by the detailed description of the] preferred embodiment.

We claim:

"1. A power transmission unit comprising a normally, driven shaft, a normally driving shaft adapted to drive said driven shaft through a clutching mechanism compris- 'mediate shaft with said driven shaft, said automatic clutch being biased to engagement by resilient means and being self-biased to slipping disengagement against said resilient means by a predetermined torque application to said driving shaft from said driven shaft, said resilient means providing a variable force that diminishes appreciably as said automatic clutch moves from engagement to disengagement to lessen slipping wear.

2. A power transmission unit comprising a normally ,driVen shaft, a normally driving shaft adapted to drive said driven shaft through a clutching mechanism com-' prising an intermediate shaft, an automatic clutch having engageable members for connecting said intermediate shaft with said driving shaft, and a manually controllable clutch having engageable members for connecting said intermediate shaft with said driven shaft, said automatic clutch being biased to engagement by resilient means and against engagement by a self-generated hydraulic force, said automatic clutch being self-biased to slipping disengagement against said resilient means by a predetermined torque application to said driving shaft from said driven shaft, said resilient means providing a variable force that diminishes appreciably as said automatic clutch moves from engagement to disengagement to lessen slipping wear, said hydraulic force being less than said variable force during engagement and disengagement but not subject to appreciable decrease during disengagement whereby slipping wear is further lessened.

3. A power transmission unit comprising a normally driven shaft, a normally driving shaft adapted to drive said driven shaft through a clutching mechanism comprising an intermediate shaft, an automatic clutch having engageable members for connecting said intermediate shaft with said driving shaft, a manually controllable clutch having engageable members for connecting said intermediate shaft with said driven shaft, said automatic clutch being biased to engagement by resilient means and being self-biased to slipping disengagement against said resilient means by a predetermined torque application to said driving shaft from said driven shaft, and means actuated by said automatic clutch when it disengages on said predetermined torque application to disengage said manually controllable clutch whereby said automatic clutch may reengage.

4. A power transmission unit comprising a normally driven shaft, a normally driving shaft adapted to drive shaft with said driving shaft, a manually controllable clutch having engageable members for connecting said intermediate shaft with said driven shaft, said automatic clutch being biased to engagement by resilient means and against engagement by'a clutch generated hydraulic force, said automatic clutch being self-biased to slipping disengagement against said resilient means by a predetermined torque application to said driving shaft from said driven shaft, said resilient means providing a variable force that diminishes appreciably as said automatic clutch moves from engagement to disengagement to lessen slipping wear, said hydraulic force being less than said variable force during engagement and disengagement but not subject to appreciable decrease during disengagement whereby slipping wear is further lessened, and means actuated by said automatic clutch when it disengages on said predetermined torque application to disengage said manually controllable clutch whereby said automatic clutch may re-engage.

.5. A power transmission unit comprising a normally driven shaft, a normally driving shaft adapted to drive said driven shaft through a clutching mechanism comprising an intermediate shaft, an automatic clutch having cngageable jaw teeth for connecting said intermediate shaft with said driving shaft, and a manually'controllable clutch having engageable friction members for connecting said intermediate shaft with said driven shaft, said jaw teeth being biased to positive engagement by a spring and being helically formed so as to be self-biased out of positive engagement and into ratcheting disengagement against said spring by a predetermined torque application to said driving shaft from said driven shaft, said spring being of conical configuration and having'a cone height and thickness ratio chosen so that the spring force diminishes appreciably as said automatic clutch moves from engagement to disengagement to lessen jaw teeth ratcheting wear.

6. A power transmission unit comprising a normally driven shaft, a normally driving shaft adapted to drive said driven shaft through a clutching mechanism comprising an intermediate shaft, an automatic clutch having engageable jaw teeth for connecting said intermediate shaft with said driving shaft, and a manually controllable clutch having engageable friction members for connecting said intermediate shaft with said driven shaft, said jaw teeth being biased to positive engagement by a spring and against positive engagement by a clutch generated hydraulic force caused by said jaw teeth acting as a fluid pump, said jaw teeth being helically formed so as to be self-biased out of positive engagement and into ratcheting disengagement against said spring by a predetermined torque application to said driving shaft from said driven shaft, said spring being of conical configuration whereby its force diminishes appreciably as said automatic clutch moves from engagement to disengagement to lessen jaw teeth ratcheting wear, said hydraulic force being less than said spring force during engagement and disengagement but not subject to appreciable decrease during disengagement whereby jaw teeth ratcheting wear is further lessened.

m 12. 7, A power transmission unit comprising a normally driven shaf atiormally driving shaft adapted to drive said'drivens'haft through a clutching mechanism comprising an intermediate shaft, an automatic clutch having engageable' jaw teeth for connectingsaid intermediate shaft with'said driving shaft, and a manually controllable clutch having engageable friction members for connecting said intermediate shaft with said driven shaft, said jaw teeth being biased to positive engagement by resilient means and being helically formed so as to be self-biased out of positive engagement and into ratcheting disengagement against said resilient means by a predetermined torque application to "s'aid driving shaft from said driven shaft, and means actuated by said automaticclutch when it disengageson said predetermined torque applicationto disengagesaid manually controllable clutch having a lesser disengagement drag than said automatic clutch whereby said automatic clutch may re-engage with disengagement of said manually controllable clutch.

8, A power transmission unit'comprising a normally driven shaft, a normally driving shaft adapted to drive said driven shaftthrough a clutching mechanism comprising an intermediate shaft, an automatic clutch hav ing engageable jaw teeth for connecting said intermediate shaft with said driving shaft, and a manually controllable clutch having engageable friction members for connecting said intermediate shaft with said driven shaft, said jaw teeth being biased to positive engagement by a spring, said jaw teeth forming pumping means during ratcheting disengagement that generates a hydraulic force acting between the teeth to resist positive engagement of said jaw 'teeth, said jaw teeth being helically formed so as to be self-biased out of positive engagement and into ratcheting disengagement against said spring by a predetermined torque application to said driving shaft from said driven shaft, said spring being of conical configuration whereby its force diminishes appreciably as said automatic clutch moves from engagement to disengagement to lessen jaw teeth ratcheting wear, said hydraulic force beingjless than said spring force during engagement and disengagement but not subject to appreciable decrease during disengagement whereby jaw teeth ratcheting wear is further lessened, and meansactuated by said automatic clutch when'it disengages onsaid' predetermined torque application to disengage said manually controllable clutch, said manually controllable clutch having a lesser disengagement drag than said automatic clutch whereby said automatic clutch may re-engage with disengagement of said manwhy controllable clutch.

, References Cited in the file of this patent Great Britain June 23, 

